Penicillopepsin (EC 3.2.23.X) is an aspartic proteinase isolated from the fungus Penicillium janthinellum. In preparation for the data collection at CHESS, crystals were grown of the native enzyme and of its complexes with two cyclic phosphonate inhibitors, methyl[cyclo-7[(2R)-((N-valyl) amino)-2-(hydroxyl-(1S)-1-methyoxycarbonyl-2-phenyl-ethoxy)phosphinyloxy (PP6) and methyl cyclo[(2S)-2-[[(1R)-1-(N-(L-N-(3-methylbutanoyl)valyl-L-aspartyl)amino)- (PP7). The phosphonate inhibitors act as mimics of the transition state in the cleavage of good substrates. Cyclization of the inhibitor reduces its flexibility and lowers the entropic cost of binding to the enzyme. At CHESS, crystals were transferred to solutions of the mother liquor containing increasing amounts of the cryoprotectant, glycerol. The final solution contained 20% glycerol, 40% saturated ammonium sulfate and 100 mM sodium acetate at pH 4.6. Data from five crystals, one of the native and two of each complex, were collected at 100 K with X-rays of wavelength 0.919 [unreadable] on the ADSC Quantum-4 CCD detector at station F1. The native data were processed to 0.95 [unreadable]: 415771 data were measured for 143710 unique reflections (Rmerge = 6.8%), 88.1% complete. The data from one crystal of each complex were processed to 0.90 [unreadable] resolution. For PP6, 614135 measurements were made of 174924 unique reflections (Rmerge = 4.2%), 89.7% complete; for PP7, 459707 measurements were made of 156276 unique reflections (Rmerge = 5.3%), 80.1% complete to 0.90 [unreadable]. The three models are currently being refined against these data. The crystallographic residuals are 11.2% for the native, 11.8% for the PP6 complex and 11.0% for the PP7 complex.